This group investigates the molecular mechanisms that determine the intracellular localization and sorting of integral membrane proteins in the endocytic and secretory pathways. This past year, our efforts have been primarily directed towards identifying and characterizing proteins that recognize cytoplasmic sorting signals. We had previously demonstrated that tyrosine-based sorting signals that mediate internalization from the cell surface and lysosomal targeting interacted specifically with the medium chains, mu1 and mu2, of the clathrin-associated adaptor complexes, AP-1 and AP-2. We have now conducted a detailed characterization of the factors that determine the specificity and the avidity of the signal-adaptor interactions. The requirements for interaction gleaned from these studies provide a mechanistic explanation for the involvement of tyrosine-based signals in various sorting processes. We have also conducted studies on the organization of the mu2 gene and the structure of the mu2 protein. A major accomplishment this past year was the description of a novel, ubiquitously-expressed adaptor complex named AP-3. The subunits of this complex were identified and named sigma3, mu3, beta3A-adaptin and delta -adaptin. This complex also recognizes tyrosine-based sorting signals and may be involved in sorting proteins for transport between early and late endosomes. Consistent with this interpretation, altered expression of delta-adaptin in Drosophila garnet mutants resulted in defective biogenesis of pigment granules, organelles with endosomal-lysosomal characteristics.